Polylactide films

ABSTRACT

The invention relates to polylactide films the mechanical properties of which, in particular bursting strength, elongation and tearing strength, have been improved by adding to the polymer certain plasticizers. These plasticizers are preferably glycerol esters.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/FI97/00143 which has an Internationalfiling date of Mar. 4, 1997 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

The invention relates to polylactide films with improved mechanicalproperties. The invention relates in particular to the use ofplasticizers to improve mechanical properties.

Polylactide, i.e. polylactic acid, which is usually prepared from lacticacid dimer, i.e. lactide, has already for years been used for medicalpurposes, for example in the manufacture of surgical sutures, fordegradable bone nails, and for controlled release of drugs. The use ofpolymers for packaging materials and for other bulk products has so farbeen limited by the high price of polymers and their susceptibility tobreaking down during technical processing. It has not been economicallyprofitable to produce and treat in a manner intended for medicalapplications a polymer intended for bulk products.

In recent years, interest in biodegradable polymers, i.e. biopolymers,has greatly increased, and many companies have made efforts to launch onthe market packaging materials, hygiene products, sacks and films foragricultural purposes, and sacks for waste. In particular, various filmshave gained importance.

The manufacture of films, in particular film blowing, by no meansconstitutes simple technology, and so far commercial biopolymers havenot been successful in this area. The mechanical and optical propertiesof film and, for example, its sensitivity to moisture, vary widely.

Polylactides, or condensation polymers which are based on lactic acid,are for many reasons a very attractive group of biopolymers. Theirprincipal degradation product, lactic acid, is a product common innature, it is not toxic and is used widely in the food andpharmaceutical industries. A high molecular weight polymer can beproduced by ring-opening polymerization from lactic acid dimer, lactide.Lactic acid is optically active, and thus its dimer appears in fourdifferent forms: L,L-lactide; D,D-lactide; L,D-lactide (mesolactide);and a racemic mixture of L,L- and D,D-lactides. By polymerizing theseeither as pure compounds or at different blend proportions, polymers areobtained which have different stereochemical structures affecting theirresilience and crystallinity and, consequently, also their mechanicaland thermal properties. The obtained polymers are usually hard andoptically clear, but not as such usable, owing to certain problems.

Upon forming, polylactide is in equilibrium with its monomer, lactide.This has sometimes been deemed to be advantageous, since monomers andoligomers may act as plasticizers of the polymer, but it also leads torapid hydrolysis and causes problems of adhesion in the processing ofthe polymer. Furthermore, the presence of the monomer lowers thermalstability during melt processing. In general the residual lactide mustbe removed from the polymer. An acceptable lactide content is below 2%,preferably below 1%. Various removal methods, such as evaporation, havebeen disclosed.

The breaking down of polymers during processing can be reduced by theremoval of the residual lactide, the maintenance of the water content ata low level (below 200 ppm) or by the addition of commercial stabilizers(WO 94/07941, Cargill). In terms of film blowing, an advantageous methodis to mix certain peroxides with the polymer, whereby the melt strengthof the polymer will be sufficient for film blowing (FI945964, FI945264,Neste).

Polylactide has excellent optical properties and a high tensilestrength, but it is rigid and brittle, and its elongation values arelow, as are its tearing strength and bursting strength (dart-drop).Attempts have been made to plasticize the films by using variousplasticizers. Battelle's -patent publication WO 92/04493 mentionsleaving monomers and oligomers in the polymer for plasticizing purposes.Cargill, WO 94/07941, has used various commercial plasticizers, inparticular Citroflex plasticizers manufactured by Morflex, in order tolower the glass transition temperature. However, the results have beenmodest.

However, in many film products, such as shopping bags and waste bags,bursting strength, elongation and tearing strength are especiallyimportant properties. For example, shopping bags must not break evenwhen sharp comers impinge against the film.

It has now been observed, surprisingly, that by using certain types ofplasticizers, available even commercially, it is possible to affect notonly the glass transition temperature of a polymer but also itselongation and its mechanical properties. In was observed in particularthat by using certain plasticizers, or plasticizer/filler combinations,the bursting strength (dart-drop) and the tearing strength could beimproved considerably. Usually the elongation at break of untreatedpolylactide is approx. 2-5%; that of a polylactide according to theinvention is at least 200%. The bursting strength in proportion to thefilm thickness is usually 0.5-1.0 g/μm, i.e. in practice a weight below20 g, which is the lowest weight in the standard test, will burst thefilm. For a film according to the invention, the bursting strength is upto approx. 10-30 g/μm. As a consequence of these improved properties,the use of polylactide films also for demanding purposes requiring goodmechanical properties is facilitated. Bursting strength values of 3-10g/μm are sufficient in films subjected to smaller stress, such as breadbags and other bags intended for food packaging, but are not sufficientfor, for example shopping bags.

The polylactide used in the invention can be made from L-, D- orD,L-lactide, or blends thereof, by any polymerization process.Copolymers or polymer blends may also used, but this is by no meansnecessary for the functioning of the invention. The use ofpoly-L-lactide is especially advantageous. The weight-average molecularweight (M_(w)) of the polymer according to the invention is approx.20000-400000, preferably 40000-200000. This corresponds to anumber-average molecular weight (M_(n)) of approx. 10000-20000,preferably 10000-100000.

Polylactide films can effectively be tailored according to the intendeduse by the selection of a suitable plasticizer and, when needed, afiller.

Suitable plasticizers include many commonly available commercialplasticizers, such as mono- and polycarboxylic acid esters, polymericpolyesters, polyalkyl ethers, and glycerol and glycol esters. Blends ofvarious plasticizers can also be used. The suitable plasticizer amountsare 10-30% by weight, preferably 15-20% by weight.

Glycerol esters such as glycerol triacetate and glycerol tripropionateare especially suitable plasticizers. Also various polymericplasticizers such as adipate derivatives are suitable.

The fillers used may be any conventional inorganic or organic fillers,such as calcium carbonate, kaolin, mica, talc, silica and zeolite. Thesuitable filler amount may be 0.1-10% by weight, depending on theproduct. The purpose of the filler is to serve as an adhesion inhibitorand thereby facilitate, for example, the splitting of a tubular film.

When necessary, other conventional additives, such as colorants, can beused. For example, shopping bags are most often white and in that casetitanium dioxide can be used for producing a white film.

Plasticizers and, when so desired, fillers and other additives, aremixed with the polylactide, before film blowing, by a conventional meltmixing method, for example in a double-or single-screw extruder or in abatch mixer.

As noted in patent applications FI935964 and FI945264, in polymerstabilization it is possible to use many even commercially availableorganic peroxy compounds, in particular those from which acids areformed as degradation products. Peroxides acting as stabilizers arecharacterized by a short half-life, preferably below 10 s, but mostpreferably below 5 s. Examples which can be given of suitable peroxidesinclude dilauroyl peroxide (half-life at 200° C. 0.057 s),tert-butylperoxydiethylacetate (0.452 s), t-butylperoxy-2-ethylhexanoate(0.278 s), tert-butylperoxyisobutyrate (0.463 s) andtert-butylperoxyacetate (3.9 s), tert-butylperoxybenzoate (4.47 s) anddibenzoylperoxide (0.742 s). The amount of peroxide to be used isapprox. 0.05-3% by weight. The required amount depends on the peroxidecompound and above all on the desired end product.

The stabilized polymer material can be used for manufacturing films bythe blowing method, or the polymer can, of course, be used formanufacturing cast films or sheets, which usually does not set so highrequirements on the polymer.

The uses of the films include conventional uses of films, in particularthose in which the aim is to minimize the amounts of waste and toprocess waste by, for example, composting. This involves variouspackaging materials, such as pouches, films, shopping bags and hygieneproducts, such as diapers and various agricultural films.

Sheets made from polylactide can be used as various packaging trays orcovers or, for example, in agricultural use as cultivation trays orpots.

The invention is described in treater detail with the help of thefollowing examples.

The polylactide used in the experiments was made by ring-openingpolymerization from L-lactide with the aid of a stannium octoatecatalyst, the molecular weight M_(w) was approx. 100000-160000. Thepolylactide was manufactured by Neste Oy.

Before the adding of the plasticizers, residual lactide was removed fromthe polylactide to a content below 1%, and the polylactide wasstabilized with a 0.1% t-butyl peroxy-benzoate (Triganox C, manufacturerAkzo Chemie) in accordance with patent application FI945964. The polymerwas extruded into a band and was cut into pellets. The pellets wereprocessed, without additional modification or after the modificationsdescribed in the examples, into films by using Collin film-blowingequipment. The films were tested by the following standard methods.

Melt index: ASTM D 1238

Tensile strength at break (50 and 500 mm/min): ASTM D 882 (ISO 1184)

Elongation at break (50 and 500 mm/min): ASTM D 882 (ISO 1184)

Tearing strength (Trouser): ASTM D 1938 (ISO 6383/1)

Tearing strength (Elmendorf): ASTM D 1938 (ISO 6383/2)

Bursting strength (Dart drop): ISO 7765-1A

The glass transition temperatures were determined using DSC(Differential Scanning Calorimetry) equipment. In the tensile strengthat break and elongation at break tests, the lower speed was used for themore brittle polymers.

EXAMPLE 1

15% by weight of an acetyl-n-butyl citrate plasticizer (Citroflex A-4,manufacturer Morflex) was added to the polymer. The mixing was carriedout in a double-screw extruder and the temperature profile used was25-190-200-190-190-190-195-200° C. A film was made from the obtainedpellets by the blowing method. To facilitate the handling of the blownfilm, talc was also added in an amount of 1% by weight. The polymerbecame plasticized, the glass transition temperature T_(g) was 32.2 °C., the elongation was 290/270% (MD/TD), and the tensile strength atbreak remained good, 45/43 MPa. However, the bursting strength was only52 g for a film 46 μm thick and the tearing strength was 0.65/0.84 N fora film 33/38 μm thick (MD/TD).

EXAMPLE 2

Polyethylene glycol (PEG) is water-soluble, and it is generally regardedas biodegradable. It has also been accepted for food contact (FDA).Commercial PEG grades having different molar mass values were mixed withpolylactide in the same manner as in Example 1, and blown films weremade from the polymer. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Plasticization of PLLA with polyethylene glycol grades having                   different molar mass values                                                                               Elongation                                            at break Tensile strength                                                   T.sub.g MFR.sub.2 50 mm/min at break                                        PEG p-% ° C. g/10 min MD/TD, % MD/TD, MPa                            ______________________________________                                        600   10      31     6.5-9.5                                                                              275/310  46/39                                      1350 20 38 20-30 215/270 48/23                                                2000 20 41 25-35 330/360 33/28                                                4000 20 38 75-80 220/340 65/25                                                8000 10 37 3.5-5.0 230/300 44/40                                            ______________________________________                                    

Polyethylene glycol plasticizes polylactide well. Soft films were alsoobtained, and the tensile strengths improved considerably. However,after a few days or weeks, PEG began to migrate out of the films, andthus the usability of the plasticizer did not prove to be good.

EXAMPLE 3

Triacetin manufactured by Bayer is glycerol triacetate, and it was usedin this example as a plasticizer for polylactide. Triacetin has alsobeen accepted for food contact. Polylactide blends modified withTriacetin plasticizer were prepared and tested. The blends and filmswere made as described above. The plasticizer amounts and the mineralsused, as well as the test results, are shown in Table 3.

                  TABLE 2                                                         ______________________________________                                                                        Tensile Tearing                                     strength at strength                                                        Amount Elongation break (Trousers                                           Tria-  of at break 50 mm/min tear)                                            cetin  fillers, 50 mm/min MD/TD, MD,                                          wt. % Filler wt. % MD/TD, % MPa N/mm                                        ______________________________________                                        10    --        --      246/250 47/47   5.7                                     14 TiO.sub.2 /SiO.sub.2 / 2/3, 5/3 110/140 30/15 1.9                           Talc                                                                         15 SiO.sub.2 5 196/250 39/50 3.9                                              16 TiO.sub.2 /Talc 1.5/2 240/270 42/36 1.3                                    20 TiO.sub.2 /SiO.sub.2 3/5 -- 49/18 100                                    ______________________________________                                    

The optiumum properties are dependent on the correct proportions of theplasticizer and the filler

EXAMPLE 4

More polylactide blends modified with Triacetin plasticizer wereprepared and tested. The blends were prepared as described above, butthe use of various minerals as anti-adhesion agents was in particularoptimized in order that tubular blown films could be opened more easily.The amount of Triacetin used was 15% by weight. The results are shown inTable 3.

                                      TABLE 3                                     __________________________________________________________________________    Plasticization of PLLA with Triacetin plasticizer by using                      additionally mineral fillers                                                         Elonga-                                                                            Tensile                                                             tion at strength    Tearing                                                   break at break   Bursting strength                                            500 mm-/ 500 Film Bursting strength-/ (Elmen-                                 min mm/min thick- strength film dorf)                                         MD/ MD/TD, ness W (50%) thickness MD/                                       Mineral Wt. % TD, % MPa μm g g/μm TD, N                               __________________________________________________________________________    Talc 2   241/224                                                                            48/46                                                                              48 600  12.5 1.8/3.3                                         Talc 5 234/231 42/43 50 644 12.9 4.4/5.3                                      Talc + 7 + 2 221/227 44/45 54 725 13.4 5.1/6.2                                TiO.sub.2                                                                     CaCO.sub.3 2 227/236 40/46 55 181 3.3 1.1/1.4                                 CaCO.sub.3 5 221/229 38/39 48 715 14.9 1.4/2.0                                CaCO.sub.3  + 7 + 2 228/225 39/40 51 296 5.8 2.9/3.9                          TiO.sub.2                                                                   __________________________________________________________________________

The bursting strengths obtained were very good, even better than fortypical film materials polyethylene and polypropylene. According to thereference tests, the bursting strengths for commercial film materialswere:

Polyethylene (HDPE). 150 g (film thickness 15 μm), i.e. 10 g,μm

LDPE: 60 g (25 μm), i.e. 2.4 g/μm and

PP: 128 g (40 μm), i.e. 3.2 g/μm.

EXAMPLE 5

Films of Example 3 were investigated, which had been made of apolylactide with which there had been blended 16% Triacetin plasticizer,2% talc, and 1.5% titanium dioxide (white color). The films were testedabove and below the glass transition temperature and at roomtemperature. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Testing of plasticized PLLA films at different temperatures                                  Elongation at break                                                                         Tensile strength at break                           50 mm/min 50 mm/min                                                          Temperature ° C. MD/TD, % MD/TD, MPa                                 ______________________________________                                        0           60/90        40/38                                                  23 240/270 42/36                                                              40 200/170 26/18                                                            ______________________________________                                    

EXAMPLE 6

In addition to Triacetin plasticizer, also other esters are usable. Thenext of the glycerol esters tested was glycerol tripropionate. Theproduct used was manufactured by Chemoxy International, under the tradename Tripropionin. The blending with polylactide was carried out in thesame manner as in the previous examples. The amounts of plasticizer usedwere 10 and 15% by weight, talc was added respectively in amounts of 1and 2% by weight. The results are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                        Tensile                                                         Elongation strength   Bursting                                                at break at break   strength-/                                                500 mm-/ 500 mm/  Bursting film                                           Tripro-  MFR.sub.2  min min Thick- strength thick-                            pionin Talc g/10 T.sub.g MD/TD MD/TD, ness W (50%) ness                       wt. % wt. % min ° C. % MPA μm g g/μm                           __________________________________________________________________________    10  1   2.8-3.7                                                                           42.4                                                                             9/8  36/6 50  <50  <1                                            15 2 5.6-7.2 28.8 237/234 48/45 50 600 12                                   __________________________________________________________________________

EXAMPLE 7

Polymeric plasticizers were tested, their advantage being their FDAacceptance for packaging and adhesive application. The products used inthe tests were Santicizer products manufactured by Monsanto, which arebased on polymeric adipates except Santicizer 160, which is benzyl butylphthalate. The blending and film making were carried out as previously.The results are shown in Table 6.

                                      TABLE 6                                     __________________________________________________________________________                  Elonga-                                                                           Tensile                                                           tion at strength  Bursting                                                    break at break Thick- strength Tearing                                    Santi- Amount  Amount MD/ MD/TD ness W strength                               cizer wt. % Filler wt. % TD, % MPa μm (50%) g N                          __________________________________________________________________________    160                                                                              15  Talc                                                                             1   281/278                                                                           42/42                                                                              40  33   0.39 (E)                                        160 20 Talc 2 217/252 48/42 60 >1000                                          430 15 Talc 2.5 233/240 44/48 30 <20                                          431 18 Talc/ 2/2 246/249 47/52 30 246                                           TiO.sub.2                                                                   438 15 -- -- 260/280 48/48   2.7 (T)                                          438 20 SiO.sub.2 3 250/260 44/35                                            __________________________________________________________________________     (T) Trousers tear, (E) Elmenforf                                         

What is claimed is:
 1. Plasticized polylactide films, wherein thebursting strength in proportion to the film thickness is above 5 g/μmmeasured according to standard ISO 7765-1A and the elongation at breakis above 200% measured according to standard ISO
 1184. 2. Plasticizedfilms according to claim 1, wherein the polylactide is a stabilizedL-polylactide (PPLA) and its monomer content is below 2% by weight. 3.Plasticized films according to claim 1, wherein the polylactide isblended with 10-30% by weight of a plasticizer selected from the groupconsisting of mono- and polycarboxylic acid esters, polymericpolyesters, polyalkyl ethers, glycerol esters and glycol esters. 4.Plasticized films according to claim 1, wherein the plasticizer is aglycerol ester.
 5. Plasticized films according to claim 1, wherein theglycerol ester is glycerol triacetate or glycerol tripropionate. 6.Plasticized films according to claim 1, wherein the plasticizer is apolymeric adipate.
 7. Plasticized films according to claim 1, whereinthe plasticizer is a blend of two or more plasticizers.
 8. Plasticizedfilms according to claim 1, wherein one or several mineral fillers areadditionally blended with the polylactide.
 9. Plasticized filmsaccording to claim 1, wherein the polylactide is blended with 15-20% byweight of a plasticizer selected from the group consisting of mono- andpolycarboxylic acid esters, polymeric polyesters, polyalkyl ethers,glycerol esters and glycol esters.